Exemplary embodiments of the present invention relate to a method for operating a motor vehicle internal combustion engine with an exhaust particle filter.
Exhaust particle filters are used for filtering out particles contained in the exhaust, in particular in vehicles with a diesel engine. These filters are usually designed as wall-flow filters with parallel inlet and outlet channels that are closed alternately at opposing ends and channels separated from each other by porous channel walls.
Soot particles filtered out of the exhaust gradually accumulate in the exhaust particle filter, which results in a gradual increase in the flow resistance of the exhaust particle filter. Associated with this is an increase in the exhaust back-pressure caused by the exhaust particle filter, which in turn results in impairment of the engine operation. The soot loading of the exhaust particle filter can be reduced again and the flow behavior be improved by oxidation of the soot particles, for example by burning-off with oxygen present in the exhaust.
Since the ash consists of non-combustible inorganic substances, accumulation of ash that cannot be removed in the soot oxidation operations also contributes to the exhaust particle filter loading. German Patent document DE 101 54 261 A1 describes a method that makes it possible to distinguish between the ash accumulated in the exhaust particle filter and the loading caused by the soot particles accumulated in the exhaust particle filter by determining the pressure drop over the exhaust particle filter. This method makes it possible to determine the success of a soot burn-off operation. A gradual increase in the flow resistance due to the increasing ash loading cannot, however, be prevented by the method, and ultimately leads to the exhaust particle filter becoming unusable.
Since it is generally assumed that the ash loading of the exhaust particle filter cannot be reduced during operation of the vehicle, and in particular by conventional soot oxidation operations, German Patent document DE 41 34 949 A1 discloses removal of the ash accumulated in the exhaust particle filter by blowing out opposed to the normal direction of flow of the exhaust while the vehicle is being serviced. Thus, a flow resistance that is impermissibly increased due to the ash loading is reversed and the exhaust particle filter is available for further use. The method is, however, relatively costly and requires interruption of the driving operation.
Exemplary embodiments of the present invention reduce an increase in the exhaust back-pressure of an exhaust particle filter caused by storage of ash.
In the method according to the invention, an amount of ash accumulated in the exhaust particle filter and an amount of soot accumulated in the exhaust particle filter is continuously determined. In such case, an ash increase value that characterizes an increase in the amount of ash is determined for the amount of ash. This ash increase value represents the increase in the accumulated amount of ash that has taken place within a pre-set or pre-settable time interval. If pre-set conditions for the operation of the internal combustion engine are present, this operation is changed over to a special operating mode for performing an ash detachment and transportation operation, in which operating variables of the internal combustion engine are set such that on the exhaust entry side in the exhaust particle filter a pre-settable minimum exhaust flow speed results at which detachment of ash attached to the channel walls of the exhaust particle filter and transportation of detached ash in the direction of the respective inlet channel end is made possible. In such case, the pre-set conditions for changing over the internal combustion engine operation comprise dropping below a pre-settable soot amount limit value for the amount of soot and exceeding a pre-settable ash increase limit value for the ash increase value. The ash increase value is reset with termination of the special operating mode.
The invention is based on the surprising discovery that the flow resistance caused by ash accumulation is reduced if the ash particles are accumulated substantially at the end of a respective inlet channel of the exhaust particle filter. On the other hand, the flow resistance is comparatively great if the ash particles are accumulated on the channel walls of the exhaust particle filter distributed substantially across the length of the channel. Therefore, the increase in the flow resistance brought about over the course of the operating time by ash accumulation can be reduced if ash attached to the channel walls is detached and transported to the end of the channel by virtue of the ash detachment and transportation operations according to the invention. It was furthermore recognized by the inventors that the detachment of ash attached to the channel walls is facilitated if the soot loading, i.e., the amount of soot accumulated in the exhaust particle filter, is comparatively low, in particular if it falls below a pre-settable amount of soot limit value of approximately 2 g per liter of filter volume, preferably 1 g/l.
Further, it was recognized by the inventors that the detachment of ash attached to the channel walls is facilitated if the amount of ash attached to the channel walls has not already grown too greatly, i.e., the ash increase value does not exceed a limit value of 5 g/l, preferably 3 g/l and particularly preferably 1 g/l. If these conditions are met, by setting an increased exhaust flow speed on the entry side of the exhaust particle filter the ash accumulated over the length of the channel can be detached from the channel walls and transported in the direction of the respective end of the channel. This causes the flow resistance of the exhaust particle filter caused by the ash accumulation to be reduced, and the period of use of the exhaust particle filter to be extended. The increased exhaust flow speed is brought about, if necessary, by changing over the normal internal combustion engine operation into a special operating mode. The increased exhaust flow speed in such case is more than 10 m/s, preferably more than 15 m/s and particularly preferably more than 20 m/s at the channel entrance. Depending on the level of the adjustable exhaust flow speed, the special operating mode is maintained for a greater or lesser amount of time. With termination of the special operating mode, the determined ash increase value is reset, in particular to zero.
In order to achieve the low amount of soot required for a successful ash detachment and transportation operation, in one embodiment of the invention, upon exceeding the ash increase limit value, a soot regeneration operation is started in which the amount of soot is reduced by oxidation if the amount of soot exceeds the soot amount limit value. The soot regeneration operation can be dispensed with if the amount of soot, upon exceeding the ash increase limit value, is lower than the soot amount limit value anyway.
In such case, in a further embodiment of the invention, in a soot regeneration operation, a reduction in the amount of soot takes place substantially by oxidation with nitrogen dioxide. For this, the engine operation is changed over to an NOx-rich combustion, which can be achieved, for example, by reducing the exhaust recirculation rate and/or by measures to increase the combustion chamber temperature. Nitrogen oxides (NOx) emitted by the engine predominantly in the form of nitrogen monoxide (NO) are oxidized on an oxidation catalyst preceding the exhaust particle filter to form nitrogen dioxide (NO2). NO2 flowing into the exhaust particle filter oxidizes the soot particles, which consist predominantly of carbon, even at moderate temperatures from 250° C. upwards to form gaseous carbon monoxide (CO) and/or carbon dioxide (CO2), which are discharged from the exhaust particle filter as gaseous substances with the exhaust, as a result of which the amount of soot reduces correspondingly. For optimum progress of the soot regeneration with NO2, approximately 350° C. is set for the exhaust temperature and a mass ratio of NOx and the particle emission of approximately 50 or more. If the ash increase value exceeds the ash increase limit value, then prior to performing the ash detachment and transportation operation a soot regeneration operation by oxidation with NO2 is started already upon exceeding a low soot amount limit value of approximately 0.5 g/l.
In a further embodiment of the invention, in a soot regeneration operation, a reduction in the amount of soot takes place substantially by oxidation with oxygen. In such case, higher temperatures than for a soot regeneration operation with nitrogen dioxide are necessary. Preferably, exhaust temperatures in the range between 550° C. and 700° C. are set on the entry side of the exhaust particle filter. This can be achieved by enriching the exhaust with hydrocarbons and exothermic oxidation of the hydrocarbons at the oxidation catalyst preceding the exhaust particle filter. A soot regeneration operation by oxidation with oxygen is started before carrying out the ash detachment and transportation operation preferably at amounts of soot or soot amount limit values of about 1 g/l.
In a further embodiment of the invention, in a soot regeneration operation, the amount of soot accumulated in the exhaust particle filter is reduced at least to the soot amount limit value. Preferably, the amount of soot is reduced to approximately zero in a soot regeneration operation directly preceding the ash detachment and transportation operation. Preferably, the soot loading is continuously calculated by a calculation model with estimation of the soot oxidation rate during the soot regeneration operation as well.
In a further embodiment of the invention, changing over of the internal combustion engine operation to the special operating mode takes place within a short time interval including the termination of the soot regeneration operation. This prevents a loosening, brought about by the soot removal, of a composite of deposited ash particles from being reversed again by renewed introduction of soot. It is particularly preferred if the special operating mode is started shortly before termination of the soot regeneration operation, if approximately 90% of the total soot to be removed has been oxidized and removed. However, provision may also be made to switch into the special operating mode within an amount of time of up to about 5 min after termination of the soot regeneration operation.
In a further embodiment of the invention, for determining the amount of ash an introduction of ash forming substances into the exhaust which is caused by one or more of the following variables is taken into consideration:                lubricating oil consumption,        fuel consumption,        combustion air consumption and        engine wear.        
In such case, an introduction of ash forming substances into the exhaust particle filter takes place, as was established, predominantly due to the lubricating oil consumption. Mineral containing oil additives, such as, for example zinc or alternatively or additionally calcium, pass into the exhaust, which results in deposition of corresponding phosphates or sulfates that have to be classed with the ash. Provision may therefore be made to take into consideration exclusively the lubricating oil consumption for determining the amount of ash. It is, however, advantageous to also take into consideration one or more of the other ash sources for determining the amount of ash.
In a further embodiment of the invention, the amount of soot is determined in a computational soot loading model in which an introduction of soot into the exhaust particle filter and oxidation of soot accumulated in the exhaust particle filter is estimated and balanced, wherein at least the following are taken into consideration                soot emission of the internal combustion engine,        concentration of oxygen and nitrogen dioxide in the exhaust on the entry side of the exhaust particle filter and        temperature of the exhaust particle filter. Preferably, the amount of soot both during normal operation of the internal combustion engine and in a soot regeneration operation are calculated. In such case, a separate soot burn-off model can be stored for the latter case.        
Further advantages, features and details of the invention will become apparent from the following description of preferred examples of embodiment and with reference to the drawings. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the descriptions of the figures and/or shown in the figures alone can be used not only in the combination specified in each case, but also in other combinations or alone, without departing from the scope of the invention.